Water heating systems are in common use in most domestic dwellings and generally comprise a hot water tank, a water inlet conduit for delivering cold water to the tank from a city water system or a well, and a water outlet conduit for delivering heated water from the tank to a demand location such as a faucet located within the dwelling. Means are provided to automatically heat the water in the tank in response to the entry of cold water into the tank upon removal of hot water from the tank to satisfy a demand from a faucet or the like. Whereas these prior art hot water systems have been generally satisfactory, they do allow substantial heat loss in the system by allowing migration of heated water from the tank upstream into the water inlet conduit and downsteam into the water outlet conduit. Various systems have been proposed in an attempt to curtail or eliminate migration of the tank hot water into the cold water inlet and hot water outlet conduits but these systems have either been unduly complicated, resulting in a high initial installation cost and high maintenance costs, or, if relatively simple and inexpensive, have been unsatisfactory in performance and/or have not provided the versatility needed to satisfy the varying demands of different types of hot water tank installations.